Kazutaka Mitsuishi

While vertical gallium nitride (GaN)‐on‐silicon architectures promise a transformative leap in cost‐effective power electronics and high‐resolution micro‐LEDs, their deployment remains bottlenecked by the high electrical resistance of conventional epitaxial buffer layers. Here, a universal and straightforward sputtering‐based strategy is presented to realize high‐quality GaN epitaxial films on Si(111) substrates characterized by exceptionally low vertical resistance, ohmic behavior, and robust thermal stability. This technique centers on the in situ formation of a sub‐nanometer (∼0.5 nm) silicide‐based template via rapid thermal annealing—a method demonstrating unprecedented versatility across 25 different metallic species. Scanning transmission electron microscopy (STEM) reveals that a unique amorphous‐like interlayer (AL‐IL) effectively accommodates lattice mismatch and relaxes epitaxial strain. These AL‐IL templates further serve as high‐performance platforms for metalorganic chemical vapor deposition (MOCVD) overgrowth, successfully bridging the gap between scalable, low‐cost fabrication and device‐grade vertical performance.

Rational design of high-performance bimetallic nanocatalysts requires an understanding of the unique atomic structures governing their performance. This study focused on the Re-Ge/TiO₂catalyst, which exhibits high performance for carboxylic acid hydrogenation. We analyzed structural and electronic-state changes during its multistep preparation involving calcination in air, hydrogen reduction, and oxidative stabilization, to elucidate structural factors responsible for the high performance. Therefore, we used complementary in-situ X-ray absorption fine structure analysis and ex-situ scanning transmission electron microscopy (STEM). The STEM imaging utilized an air-free transfer holder to track individual nanoparticles and address two issues: the challenge of distinguishing true structural changes from variations among different particles, and electron beam damage associated with long exposure times. It was revealed that the highly active state after hydrogen reduction originates from approximately 1-nm crystalline alloy nanoparticles. The nanoparticle structure was a unique, low-energy, face-centered cubic fragment, verified by density functional theory calculations. Furthermore, quantitative STEM analysis demonstrated that the nanoparticles were a random alloy of Re and Ge. This atomic-level mixing, as detected by the spatially averaged X-ray absorption, considered to stabilize the metallic state, resulting in electron-rich Re. These findings provide a new strategy for designing high-performance nanocatalysts and establishes the correlative methodology as a way to identifying performance-determining factors in complex nanomaterials.

Protonation of polycrystalline garnet H _5.2 Li _1.3 La ₃ Zr _1.5 Ta _0.5 O ₁₂ alters local Li environments and dominates ionic transport, while Li ⁺ mobility remains largely frozen, highlighting how Li ⁺ /H ⁺ exchange affects conductivity in garnet-type electrolytes.

Electron ptychography offers visual evidence of a key initial degradation step: atomic-scale loss of Cu ions from eight-membered-ring pores.

Cu‐exchanged SSZ‐13 zeolite (Cu‐SSZ‐13), a dynamic catalytic system, plays a critical role in the selective catalytic reduction of nitrogen oxides with ammonia (NH ₃ –SCR). Catalytically active Cu ions exhibit activity through the adsorption and desorption of ammonia. Although the actual reaction involves complex interactions, clarifying Cu speciation based on water‐Cu interactions provides a fundamental baseline for understanding the active site dynamics. Although x‐ray diffraction and absorption have provided insights into the dynamic changes of Cu sites, their interpretation remains limited to average structures, leaving atomic‐scale local structures unresolved. Here, we directly visualized the hydration‐induced migration of Cu ions using electron ptychography, which enabled low‐dose imaging with atomic resolution. Under hydrated conditions corresponding to catalytic environments, Cu ions were observed to migrate from double six‐membered ring sites to eight‐membered ring sites. Although these hydrated Cu sites were largely consistent with those predicted using theoretical models, sub‐angstrom deviations were detected, suggesting the presence of a previously unrecognized Cu site. Revealing the local structure and dynamics of guest metal ions not only provides critical insights for optimizing catalytic performance but also advances our fundamental understanding of the mechanisms of catalytic activation and degradation, thereby opening new avenues for developing advanced zeolite catalysts.

Layered cathodes are essential for achieving a high energy density in rechargeable lithium-ion batteries, but they suffer from capacity fade due to local crystal phase transitions during charge-discharge cycles. Understanding these phase transitions is essential to minimizing their effects, but their nanometer size and sensitivity to experimental conditions make observation difficult. Here, we visualized nanometer-scale crystal phases within an epitaxial LiCoO₂ cathode after 100 charge-discharge cycles by matching experimental and simulated cepstra derived from scanning nanobeam electron diffraction. While the LiCoO₂ bulk remained a layered structure, spinel- and rocksalt-type phases were observed within 3 nm of the cathode-electrolyte interface. The developed method achieved a spatial resolution of 1.5 nm across a field of view of several hundred nanometers with minimal electron beam damage. The cepstral matching analysis offers valuable insights into the interfacial phase transitions and will aid in the design of high-performance lithium-ion batteries.

Dynamics in liquids and glasses can be assessed using X-ray photon correlation spectroscopy or electron correlation microscopy, which involves measuring the temporal changes in diffraction patterns. Two methods are commonly used to evaluate these temporal changes: one-time correlation function or two-time correlation function. However, the specific characteristics of these methods have not been thoroughly studied. In this study, we investigated the differences between these methods and found that the two-time correlation function can measure dynamics for longer periods than the method relying on the one-time correlation function. Additionally, we demonstrated that the two-time correlation function exhibits a weak dependence on the amount of dose applied.

Magnetic structure analyses of a material and its associated spin configurations are important issues not only in the fields of solid-state physics, inorganic chemistry, and spintronics but also in other fields such as materials science, engineering and electronics industries. The magnetic fine structure is generally determined by neutron scattering, X-ray magnetic circular dichroism, spin-polarized scanning tunneling microscopy, magnetic exchange force microscopy and electron microscopy. Among them, transmission electron microscopy (TEM) is a powerful tool capable of site-specific analysis of the magnetic structure inside materials. Development of hardware-type aberration correction in electron microscopy has enabled local structural observations with atomic resolution as well as chemical and vibration analysis. As for magnetic imaging, however, atomic-level spin configuration has been analyzed by electron energy-loss spectroscopy by placing samples in strong magnetic fields, which destroy the nature of the magnetic ordering in the samples. Although magnetic-field-free observation can visualize intrinsic magnetic fields of an antiferromagnet by unit-cell averaging, it is a challenge to directly observe the magnetic field of an individual atomic layer of a non-uniform structure. We have developed a pulse-magnetization system, to extract the magnetic signal by separating it from non-uniform electrostatic (structural) signal. This system has enabled magnetic-field observation of local areas such as layers and nanomagnets at resolution of sub-nanometres [ 1]. However, there are additional problems to increase the resolution. The magnetic signal at atomic scale is weak; therefore, to increase signal-to-noise ratio, high-resolution magnetic-field observation requires relatively longer observation time than that of a strong electrostatic signal of atoms.